The gratuitous use of stun guns in U.S. prisons results in “grave abuse” and in some cases amounts to torture, according to the United Nations special rapporteur on torture, who on Thursday called on U.S. officials to consider criminal charges against prison authorities in several southern states.

The U.N.’s Nils Melzer said video footage published this week by Reuters showed that 22 incidents of stun gun use in prisons in Ohio, Tennessee, Arkansas and Oklahoma violated the U.N.’s prohibition on cruel and inhuman punishment.

Melzer told Reuters:

In my view, all of the incidents shown in this video require independent investigation and most of them are likely to merit prosecution. Clearly gratuitous infliction of severe pain and suffering…constitutes a grave violation of human dignity and of the universal code of conduct for law enforcement officials.

Reuters documented hundreds of cases since 2000 in which stun guns, often referred to using the brand name Taser, were misused or linked to allegations of torture. And it identified 104 inmate deaths involving the weapon. More than a quarter of the 84 autopsy filings obtained by the news agency showed that they were listed as a cause or a contributing factor of death. In a majority of cases examined by Reuters, incarcerated people were already restrained by officials when they were shocked.

The U.N. is not alone in condeming widespread stun gun use. Between 2001 and 2012, more than 500 people died in the U.S. after law enforcement officials shocked them, according to a report by Amnesty International. Among law enforcement agencies, the Oklahoma City Police Department recorded the highest number of deaths during that span, followed by the Las Vegas Metropolitan Police and the Harris County Sheriff’s Department in Texas.

Said Susan Lee, America’s program director at Amnesty International:

Of the hundreds who have died following police use of Tasers in the United States, dozens and possibly scores of deaths can be traced to unnecessary force being used. This is unacceptable, and stricter guidelines for their use are now imperative.

As the American Civil Liberties Union points out, stun guns were originally intended to function as a non-lethal method of controlling violent suspects. But according to the Reuters report, corrections systems veterans say the stun gun is frequently used on people who pose no physical threat.

Reports also show that use has increased among law enforcement officials, but many police departments have failed to implement Taser training policies. Meanwhile, medical studies show that stun guns can cause rapid loss of consciousness, sudden cardiac arrest and death.